1. Field of the Invention
This invention relates to electrochemical power sources such as cells and batteries. Specifically, this invention relates to a method for making gel polymer lithium ion rechargeable or secondary cells and batteries. More specifically, the present invention relates to a single step, in situ polymerization method for making gel polymer lithium ion rechargeable cells and batteries.
2. Prior Art
The worldwide demand for portable electronic devices is growing rapidly and is responsible for the increasing need for high density and lightweight electrical energy power sources. To meet this growing demand, lithium ion secondary batteries, particularly rechargeable high energy density flat batteries containing gel polymer electrolytes, have been developed. These electrolyte chemistries comprise liquid plasticizers trapped in a polymer matrix. Gel polymer electrolytes have the following advantages compared with conventional liquid electrolytes: (a) they contain no free-flowing liquid and, therefore, the possibility of electrolyte leakage is eliminated, (b) they provide flexibility for engineering design, especially for flat and thin batteries, and (c) they are safer to use than their liquid counterparts.
An exemplary gel polymer electrolyte cell is described in U.S. Pat. No. 6,235,433 to Amano et al. This patent teaches a gel electrolyte comprising: i) a compound containing at least two polymerizable functional groups in the same molecule, and ii) one of a first through third compounds having a polymerizable functional group and containing a carbonyl group, a amido group, and an oxyalkylene group, respectively, in a vinylidene fluoride polymer. The compound containing at least two polymerizable functional groups has an unsaturated ethylene bond and, particularly, may have an acryloyl group or a methacryloyl group. Specific examples are diacrylates, triacrylates, tetraacrylates, and combinations thereof.
However, Amano et al. did not realize that a gel polymer electrolyte comprising acrylate functionalities could be improved upon with one acrylate having a relatively low functionality and another having a relatively high functionality. The former benefits reduced shrinkage, increased flexibility and adhesion, and free volume for ion conducting electrolyte flow while the latter promotes rapid polymerization.